TM 5-814-3/AFM 88-11, Volume III
of 2 inches beneath the soil surface to provide adequate disposal and purification. High water tables can be
determined by direct observation or by soil mottling. Occurrence of grey and red soil mottling phenomena
can be used to indicate periodic saturation with water. However lack of mottling does not always mean that
seasonally perched water does not occur. Looking at mottling is meaningful but direct observation is
preferable if there is any doubt.
e. Depth to impermeable soil layer or rock strata. The depth to impermeable soil or rock strata can
vary over a range (see fig 6-7 and fig 6-8). The optimum distance will vary for a given site. Sufficient area
must be available so that the effluent can move away from the mound. Otherwise, effluent will build up in
the mound and cause seepage out the toe of the mound. Climatic factors, soil permeability, slope, and system
configuration affect this distance. Slowly permeable soils require more area to remove the effluent from the
mound than do permeable soils. Frost penetration reduces the effective area for lateral movement; thus, in
warmer climates, depth requirements are not as great as for colder climates. Level sites require shallower
depths than do sloping sites, as more area is available for effluent dispersal since the effluent can move in
several directions. Less depth is required for long narrow mounds than is required for more square systems
because the square system concentrates the liquid into a smaller area.
f. Depth to 50 percent volume rock fragments. Rock fragments do not assist in purification and
disposal of effluents. They cause the effluent to be concentrated between the fragments. This may lead to
saturated flow and, thus, poorer purification. If the soil contains 50 percent rock fragments by volume in the
upper 24 inches of soil, then there is only half the soil available for purification and disposal of the effluent.
Depths greater than 24 inches must be used if the soil beneath the mound contains more than 50 percent by
volume of rock fragments. This is especially true for permeable soils over creviced bedrock and in areas
where the high water table may intersect a potable water supply.
g. Slopes. Site selection is very important. The crested site is the most desirable because the mound can
be situated such that the effluent can move laterally down both slopes. The level site allows lateral flow in
all directions but may present problems in that the water table may rise higher beneath the mound in slowly
permeable soils. The most common is the sloping site where all the liquid moves in one direction, away from
the mound. However, proper design can overcome this limitation, especially in the less permeable soils. The
mound should be placed upslope and not at the base of the slope. On a site where there is a complex slope,
the mound should be situated such that the liquid is not concentrated in one area of the downslope. Upslope
runoff should be diverted around the mound. Mounds require more stringent slope specifications than
conventional systems because of their reliance on lateral movement of effluent through the upper soil
horizons. Lateral movement becomes more important as soil permeability becomes less. Thus, on more slowly
permeable soils, the maximum allowable slopes are less. For the more permeable soils (3-29 minutes per
inch), slopes up to 12 percent should function without surface seepage because lateral movement is not so
great. For tighter soils (30-120 minutes per inch), slopes should not exceed 6 percent. For sloping sites, the
downslope distance (I) must be lengthened and the upslope distance (J) shortened. Table 6-2 may be used
for this calculation (see sample problem in appendix C).
6-14
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